1. Field of the Invention
The present invention relates to an accessory for attenuated total reflection (ATR) infrared (IR) spectroscopy for making a point analysis, line analysis, or area analysis of the surface of a sample, using infrared radiation. The invention also relates to a crystalline element used for ATR IR spectroscopy.
2. Description of the Related Art
Where a crystalline element of a higher relative index of refraction (i.e., the index of refraction of a material divided by the index of refraction of air; hereinafter referred to as the index of refraction) is brought into intimate contact with a sample of a lower index of refraction under examination, if infrared radiation is made to enter the interface between the crystalline element and the sample at an angle exceeding the critical angle, then the radiation penetrates to a certain depth into the sample and then totally reflects. Therefore, if the sample absorbs infrared radiation, then the intensity of the totally reflected light decreases, i.e., the totally reflected light attenuates, according to the intensity of the absorption. An ATR IR spectrum intrinsic to the sample is obtained by detecting the totally reflected light. An accessory for measuring the absorption of totally reflected infrared radiation analyzes such a spectrum to obtain information about the chemical composition of the surface layer of a sample having a thickness of several microns. This kind of accessory is widely used to make a surface analysis of polymeric materials of relatively low indices of refraction, such as rubber, films, and plastics.
A typical example of the prior art accessory for ATR IR measurements is shown in FIG. 23. This accessory utilizes multiple total reflections and includes a condenser mirror 1 which converges infrared radiation onto the incident surface 3 of a crystalline element 2 where total reflection takes place. The radiation enters the crystalline element 2 and totally reflects many times inside it. A sample 5 to be investigated is placed in intimate contact with the totally reflecting surface 4 of the crystalline element 2. During the multiple total reflections inside the crystalline element, the infrared radiation is totally reflected a plurality of times at the interface between the sample 5 and the crystalline element 2 to improve the sensitivity of the accessory in making the surface analysis. Infrared radiation exiting from the exit surface 6 is directed to a detector (not shown) via an objective mirror 7. The cross section of the conventional crystalline element assumes various forms including parallelogram and trapezoid as shown in FIG. 22 (1) -(3).
In recent years, strict demands have been made on micron order analyses of samples. For example, it has been required to make a surface analysis of a microscopic region having dimensions on the order of 10 microns on the surface of a sample.
In the above-described accessory performing ATR IR measurements using multiple total reflections, the infrared radiation is focused onto the incident surface 3 of the crystal. Therefore, it is inevitable that the infrared radiation is dispersed inside the crystal. Also, it is difficult to efficiently focus the infrared radiation onto the sample. For these and other reasons, it is difficult to enhance the efficiency of utilization of the quantity of light in the microscopic area under investigation. Therefore, the accessory cannot analyze areas smaller than an area having dimensions of the order of 100 microns. Even if the infrared radiation emanating only from a sample surface portion having dimensions on the order of 10 microns is selected using a slit, satisfactory ATR IR spectra cannot be obtained because of insufficiency of the intensity of the infrared radiation from the sample surface portion of interest, the insufficiency being caused by insufficient efficiency of utilization of the quantity of light. Hence it is impossible to analyze this sample surface portion.
The above-described demand for microscopic analysis may require that a line analysis on the order of 10 microns be made. The prior art accessory for ATR IR spectroscopy cannot satisfy this requirement because of the limited ability of analysis as mentioned above. In addition, the accessory is intrinsically unsuitable for the above-described requirement for making an analysis along a line, because the multiple total reflection method involves taking the average of values derived from plural sample surface portions where the plural total reflections of infrared radiation take place.
Another requirement is an area analysis on the order of 10.mu.. The prior art accessory for ATR IR measurements cannot fulfill this requirement for the same reason as in the case in which a sample surface is analyzed along a line.
In the prior art accessory utilizing multiple total reflections for making ATR IR measurements, it is customary to bring the focus of the objective mirror onto the exit surface of the crystalline element to obtain the maximum amount of infrared radiation. Thus it is impossible to make a point analysis, line analysis, and especially area analysis of the specific sample portion. Also, it is impossible to visually observe a certain point of measurement via the objective mirror, because the focus is not on the measurement point. In this way, if the function of visual observation via the objective mirror is added in the prior art, it has been useless for microscopic analysis.
With the prior art crystalline element for producing total reflections, the optical axis of the outgoing rays deviates from the optical axis of the incident rays as indicated by the broken lines in FIG. 22. This makes it impossible to dispose the condenser mirror 1 and the objective mirror 7 on the same axis, as shown in FIG. 23. Therefore, the prior art crystal cannot be applied to the conventional infrared microscope because it has condenser optics where the condenser mirror and the objective mirror are arranged on the same axis.
If this microscopic measurement by ATR IR spectroscopy is made, it is difficult to visually identify a point of interest on the sample surface. It is because the sample does not sufficiently adhere to the crystal and because the visible light penetrates less deep into the sample than infrared radiation on total reflection. In order to solve the foregoing problem, an index for locating the microscopic point of measurement is needed.